In view of the recent environmental issues and the energy saving limitation, dual sided printing is essential in a plain paper copy, a printer, or the like. In an ordinary dual sided printing system, a method of inverting the paper inside or outside the machine after printing one side, feeding the paper from a paper feeding section, and again printing the back side, is adopted. In this case, since only an exterior type inverting dual sided unit is needed, a simple configuration can be achieved as the machine configuration. However, the productivity in the case of dual sided printing, in particular, the productivity of first printing is drastically deteriorated compared with one side printing. Then, for example, systems disclosed in the Japanese patent Application Laid Open (JP-A) No. 5-35043, and the U.S. Pat. No. 5,461,470 are known. Those systems comprise a plurality of image producing devices for continuously dual sided printing.
In contrast, as a system for executing dual sided printing at the same time, the devices disclosed in the JP-A No. 11-160951, and the U.S. Pat. No. 4,714,939 are known. These devices are for transferring the image of a first surface onto the paper by inserting a paper between a photosensitive member and the intermediate transfer belt after temporarily copying a second surface image with the mirror image process applied on an intermediate transfer belt, and consecutively transferring the second surface image from the intermediate transfer belt to the paper back side at the paper separation position from the intermediate transfer belt.
However, according to the former system comprising a plurality of image producing devices for continuously executing dual sided printing, due to bulkiness of the machine and significant waste in the case of one side printing, it is used practically only in the high speed systems for business use.
Moreover, according to the latter system for transferring the image of a first surface onto the paper by inserting a paper between a photosensitive member and the intermediate transfer belt after temporarily copying a second surface image with the mirror image process applied on an intermediate transfer belt, and consecutively transferring the second surface image from the intermediate transfer belt to the paper back side at the paper separation position from the intermediate transfer belt, at the time of dual sided printing, since the intermediate transfer belt should make a round, the productivity is not high.